Disclosed herein is a method and an apparatus for measuring prism characteristics in a single prism, prism film or prism sheet. More specifically, the method and the apparatus are used for measuring the prism apex angle (hereinafter the apex angle), the prism skew angle (hereinafter the skew angle) and the refractive index of the material used in the prism.
Brightness enhancing display films generally termed “prism film” or “prism sheets” are used in liquid crystalline display devices in order to concentrate the light on a liquid crystalline layer. While the prisms in prism films or prism sheets can have any apex angle depending on the details of the backlight configuration and the desired output, it is generally desirable to employ prisms on the brightness enhancing display films that have apex angles of about 90 degrees. It is further desirable for a bisector of the apex angle of the prisms to be normal to the back surface of the brightness enhancing display film. If the bisector of the apex angle is not normal to the back surface, there is a reduction in the angular concentration of light. The angle between the bisector of the apex angle and a normal to the back surface is termed the “skew angle”. If the prism apexes are perfectly aligned relative to the sheet itself, the prism skew angle is zero.
Prism sheets are manufactured by pressing a malleable material against a prism-shaped mold. Possible manufacturing processes include melt calendaring, embossing, injection molding, compression molding, casting and curing of thermally cured resin onto a substrate, and casting and curing of UV cured resin onto a substrate For example, the mold can be an electroform which is a replica of a drum that has a negative image of a prism surface machined on its outer surface by using a turning machine such as a lathe. Many other micro-machining techniques can also be employed, including those that create a flat master, such as micro-milling, and fly-cutting. The negative image of the prism surfaces can be manufactured with a cutting tool made of hard material such as diamond. It can also be manufactured through other micro-texturing methods such as laser engraving and photolithography.
When micromachining a prism sheet using a cutting tool, a misalignment of the cutting tool during the machining can result in a defective master. The defective master produces a defective mold, which then stamps out a defective brightness enhancing display film. Defective brightness enhancing display films generally have prism apex angles or skew angles that vary from the desired values. Differences of minutes or even seconds in the apex angle or in the skew angle can affect performance.
There exist many techniques to measure the refractive index of thin films or bulk solids, but most utilize expensive analytical devices such an ellipsometer, refractometer, or a prism coupler. In addition to the considerable overhead required of these analytical devices, one limitation is that they can only measure optically flat surfaces. In particular, they cannot be used to measure the refractive indices of prism sheets that contain microstructured prisms.
There exist many analytical techniques available to characterize the geometry of microstructures, such as stylus profileometry, confocal microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). However, they are expensive and labor-intensive. They are also usually destructive and suffer from measurement artifacts related to sample handling. In the case of measurements involving contact between the prism sheet and a portion of the analytical tool (e.g., a probe), the size and shape of the probe can also add artifacts to the measurement. Such measurements also cannot fulfill the demand for highly accurate, quantitative values of the apex angle and the skew angle of the prism sheets. In microscopy, such angles are generally extrapolated by processing micrographs of cross-sections.
The quality of the calculated angles is limited by the quality of the image itself; for example, the image may be out of focus or lack sufficient resolution. It is also difficult to make a cross-section of a sample absolutely perpendicular to the prism direction, be it through microtoming, or through dragging a contact probe across the sample. The sample can become deformed by the blade or the probe, as well. Hence, such techniques cannot provide quantitative values of the apex angle or the skew angle with the degree of accuracy demanded of optical applications.
Given the demands placed on optical applications, the manufacture of optical grade microstructured films requires accurate and reproducible measurements of both material refractive index and film geometry (e.g., the apex angle and the skew angle). In the case of the prism sheets, a refractive index difference in the third decimal place may affect performance of the end product (e.g., liquid crystalline displays).
There therefore does not exist any methods that can simultaneously measure the refractive index, apex angle, and skew angle of a microstructured prism film or sheet. In order to minimize geometric defects in the prisms it is desirable to use a method that can determine whether there are geometric defects present in the prism while at the same time detecting the refractive index of the material of the prism.